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Efficiently using limited energy resources over the long term makes it more important to optimize energy consumption versus simply cutting usage. Dr. Gyuyoung Yoon, Associate Professor at Nagoya City University, has coupled Cradle scSTREAM with an air-conditioning energy management tool to account for the impact of three-dimensional effects on total energy management.

Chapter 2 Material Properties I

This chapter describes the role of material properties used in a thermo-fluid analysis. The property of a material describes its characteristics and behaviors, and is specified by a numerical value that can be either calculated or found in available textbooks or reference manuals. The value of the material property is often not a constant but can vary depending on the conditions, e.g. temperature and pressure. Some examples of material properties are shown below. Notice how a large or small value of the material property helps you understand the characteristics and behaviors of the material. All units are expressed in SI units.

2.1 Density

Density is a material property. The density of a substance is its mass per unit volume and the unit of density is kg/m3 . Consider the density of iron and expanded polystyrene as examples. Even if iron and expanded polystyrene are the same size, you can easily imagine that their masses are significantly different. The difference is caused by the difference in their densities. The density of expanded polystyrene is approximately 30 kg/m3 while that of iron is 7,870 kg/m3 .

Figure 2.1: Density difference

We often don’t consider the density of the gas and fluid streams that surround us in our daily lives. However, both air and water also have mass and density. The density of dry air (air without any water vapor in it) is approximately 1.206 kg/m3 at 1 atmospheric pressure and 20 º C. On the other hand, the density of water is 998.2 kg/m3 , nearly 1,000 times greater than the density of air. The density of an object greatly affects the amount of impact when the object is in motion. For a beach ball and a bowling ball rolling with the same velocity, the bowling ball can apply more power to an object because its density is higher and it weighs more. In the same way, when air and water flow with the same velocity, water, which has greater density, can apply more power to an object.

2.2 Viscosity coefficient

Viscosity is a material property that is specific to fluids. If you stir water and then you stir syrup, you’ll need more force to stir the syrup. This is because the syrup is thicker than water and more resistant to motion. The viscosity is used to characterize a fluid’s resistance to motion. Viscosity coefficient indicates the degree of viscosity and its unit is Pa・s.

Figure 2.2: Viscosity difference

A fluid’s viscosity tends to regulate its flow. For example, when you stop stirring water in a bucket, the water’s viscosity is what causes the water motion to gradually slow. The slowing-down starts from the outer diameter that is in contact with the wall of the bucket. The effect of viscosity on the flow of a fluid is also dependent on the fluid’s density. A lightweight (low density) fluid will be more influenced by viscosity than a heavy fluid. Kinematic viscosity coefficient expresses the relationship between a fluid’s density and viscosity. It is calculated by dividing the viscosity coefficient by the fluid density. Its unit is m2 /s.

Software CRADLE is participating and sponsoring the 14th International Conference of the International Building Performance Simulation Association (IBPSA). Building Simulation Conference 2015 (BS2015) to be conducted at Hyderabad from December 7-9, 2015. The objective of BS 2015 is to advance the practice in diverse disciplines of building energy analysis and performance simulation. For more details check the link http://www.bs2015.in/ ContraVolts Infotech Pvt Ltd. (www.contravolts.com), A Software Cradle Co., Ltd (www.cradle-cfd.com), Japan venture will represent CRADLE at the conference.

Introduction

The Basic Course of Thermo-fluid Analysis series is intended for those who have a beginning understanding of thermo-fluid analyses and want to start using thermo-fluid analysis software or have just started to use it. The information provided in this series will present fundamental thermo-fluid analysis principles that will help build a solid foundation for future learning.
With the rapid development of software and hardware products and technologies, the environment and expectations for product design and development have dramatically changed. For example, engineers used to design in 2D, whereas today, most design is done in 3D. In the past, computer simulations were often home-grown and used correlations developed from experimental data. Today commercial computer software uses 3D models and contains hundreds of thousands of elements, to simulate complex physical phenomena at the element level.
During this trend, computer-aided engineering (CAE) has become very popular in engineering. CAE tools once required specialists to use them. However, today, more engineers in the field of thermal and fluid dynamics are using computational fluid dynamics (CFD) software as part of their daily responsibilities. This shows the growing need for thermo-fluid analysis software to be a fundamental part of the engineer’s toolkit.
Some engineers, however, may find it difficult to understand the complex theories and unfriendly technical terms used in CFD. They may not be familiar with how the software works or why it works. To help address these needs, this course attempts to simplify complex thermo-fluid concepts and makes them intuitively understandable. This is done without using complicated technical expressions or equations. We hope you enjoy this course series and that the contents help you better understand thermo-fluid analyses and CFD.

Chapter 1: Thermo-fluid analyses

This chapter describes basic phenomena about fluid flow and heat transfer. It also discusses some advantages and disadvantages of using CFD software.

1.1 Thermo-fluid-related phenomena

Air and water at room temperature are characterized by the fact that they flow without any specific shape. Substances with this characteristic are collectively called fluid.

Figure 1.1: Three states of matter

A variety of fluids, such as air and water, exist on the earth. The flow of a fluid and heat transfer relate to many phenomena observed in our everyday lives.

The air flow around automobiles and airplanes highly affects their performance.

Electronic devices and electric circuits must be designed to readily release heat so the components won’t overheat.

Understanding the flow of air and heat in and around building structures is crucial for properly designing efficient air-conditioning systems to create a comfortable inside environment.

Figure 1.2: Phenomena related to the flow of a fluid and heat transfer

1.2 Advantages of CFD analyses

Prior to computational simulation, new designs were evaluated by running tests. Then, the test results and experience were used to dictate design modifications. The test results to drive design improvements use real world data, which is very reliable. However, running tests is expensive in terms of cost, time, and labor.CFD software is a computational tool that helps overcome these obstacles. CFD stands for Computational Fluid Dynamics, which is the science of using the computer to simulate how fluids and energy flow. As analysis technology and computer performance improved, CFD accuracy improved. Today many companies routinely use CFD to design their products. Some of the advantages of CFD are as follows:

The effects of different operating conditions can be evaluated without creating prototypes. Development time and total cost can be reduced.

Detailed data can be obtained even in places where experimental measurements may not be possible. Conditions can be simulated when it may be difficult or even nearly impossible to achieve data experimentally.

The flows of a fluid and energy are difficult to visualize. However, CFD software provides a medium for seeing the invisible. Visualizing the computational results makes it possible to objectively explain phenomena.

The following figures are some examples of CFD analyses.

Figure 1.3: Examples of CFD analyses

As you can see, CFD software has many attractive features. However, as with anything, it also has its weak points. For the simulation of a computationally intensive physical phenomenon, the physical model or object geometry may be simplified. Errors related to the simplification can occur. In addition, any numerical calculation by a computer always risks calculation error.
In conclusion, being fully aware of the advantages and disadvantages of using CFD software is crucial. In addition, the simulation results should be validated using measurement data to ensure the results are consistent with real-world phenomena and physics.